The invention relates to a large-area washer with an opening for inserting a fastener, which has protruding meshing elements at the surface pointing in the fastening direction, which elements increase the friction or meshing with a panel and/or sheet to be fastened.
Such a washer, which is known from the European patent application EP 02 83 184 A1 has, on its underside, as meshing elements, molded pins that are conically pointed. In another known large-area washer, meshing elements are provided that are embodied as conical points, which are also produced in conjunction with the manufacture of the washer (U.S. Pat. No. 4,987,714). In both cases, relatively thick plastic washers are at issue. The provision of other forms of meshing elements on the underside of such plastic washers which, however, may be produced only by an injection-molding process, is also known (U.S. Pat. No. 5,069,589).
In the case of one known large-area metal washer, such meshing elements are formed of brackets stamped out of the whole material of the washer and pressed downward, such that these brackets, in frontal view, are embodied in each case, in triangular shape and are oriented with a downward point (U.S. Pat. No. 4,763,456). The triangular brackets do grip relatively well into insulation or roofing sheets, which are to be fastened. However, a serious disadvantage is presented by the fact that the brackets, which are bent downward, constitute knife blades of a sort when the corresponding forces act upon the roofing sheet, thus destroying the roofing sheet. As soon as no more than a very small cut is produced in the roofing sheet, no great tensile forces are required any longer to pull the roofing sheet out from under the washer. Such tensile forces on the roofing sheet often result from the effects of the wind, particularly in the case of flat roofs.
Washers having a large area are usually used on a solid underlying structure to fasten single layer or multi-layered roofing sheets and/or an insulation layer. The forces arising between the roofing sheet and the underside of the washers are relatively high and result, in particular, when the danger exists that the roofing sheet underneath the fastener or under the washer will be pulled out due to corresponding suction stresses. This tensile stress on the roofing sheet occurs transverse to the axis of the screw, that is, parallel to the plane of the large-area washer. The retentive force of downward bent, blade-like brackets is too slight, so that the roofing sheet, which is, in each case, attached in the area of the edge, is torn out underneath the washer.
A large-area washer, which is intended to extend beyond the contacting edges of two sealing coating sheets is already known (DE-A-29 30 993). The two end regions of this washer have, in each case, a through hole, at the edges of which gripping teeth that constitute gripping organs are embodied. In this instance, relatively large holes are stamped out, such that the edge areas of these holes are bent downward as individual pointed brackets.
Such an embodiment would also not lend itself well to absorbing correspondingly large suction forces acting on the roofing sheet, due to the fact that the gripping teeth grip into the roofing sheet with an insufficient degree of strength or, if they do engage properly, function once again as blades, which cause a separation of the sheet in response to tensions parallel to the plane of the washer.